Methods and apparatuses for detecting the thickness of a sheet material or the number of sheets in a sheet material are well known in the art. One type of sensor which has been used heretofore is a capacitive sensor wherein the sheet material passing in proximity to the capacitive sensor generates a voltage dependent upon the capacitance of the sheet material.
However, capacitance is dependent upon several factors: among them are the distance of the sheet material away from the sensor, and the characteristics of the sheet material (such as thickness or the number of sheets). Thus, the capacitive sensor of the prior art cannot discriminate between a single sheet of the sheet material in proximity to the capacitive sensor versus a plurality of sheets further away from the capacitive sensor. In both cases, the capacitance reading could be the same.
One solution to this prior art problem is to have mechanical holding means to hold the sheet material at a preset distance from the capacitive sensor. However, this requires additional mechanical equipment. Further, throughput degrades because the sensor is intended to be used in an environment where the sheet material is moving rapidly and the sensor is intended to measure large volume of sheet material flowing through the system.
A second prior art solution to this problem is to place each capacitor plate of the capacitive sensor on opposite sides of the sheet material, instead of placing them on one side of the sheet material, as in the first prior art described above. In this arrangement, the movement of the sheet material away from one plate results in a movement toward the other plate, with no resultant change in capacitance. In this scheme, however, the capacitor plates must be widely separated in order to accommodate the placement of the sheet material between them. This results in a very significant decrease in the sensitivity of the capacitive sensor.
Capacitive sensors employ an AC stimulation voltage to measure the overall capacitance between their sensor plates. If the capacitor plates are more separated, the stimulation signal must be operated at an elevated frequency or strength in order to overcome the loss in capacitance caused by the increased separation. The higher frequency or stronger stimulation signals are more difficult to generate and more difficult to convey to the capacitor plates. They also require special shielding arrangements to prevent excess radiation of the stimulation signals out of the equipment.
Another means of compensation for the decreased sensitivity caused by separation of capacitor plates is to increase their size. This has a deleterious effect on the overall compactness of the apparatus however, because the plate size must be very much larger than the original size in order to achieve equal sensitivity. Further, the increased plate size results in poorer spatial resolution of smaller sheets of sheet material, as well as decreased resolution of the edges of the sheet material.
The signal generated by the sensor is compared to a fixed reference voltage. Since the reference signal is fixed, any change in the operating condition, such as humidity, which changes the signal from the sensor, but is not reflective of a change in the number of sheets or the thickness of the sheet material may result in an erroneous reading.